1. Field of Invention
The present invention relates to a method of designing an assist feature. More particularly, the present invention relates to a method of designing an assist feature, incorporated with off-axis illumination (OAI) in order to increase the process window of deep submicron.
2. Description of Related Art
Photolithography plays an important role in the semiconductor process. For each semiconductor device, patterns of all films and doping regions are decided by this step. Therefore, the numbers of photolithography required for a process, which is the needed mask number, decide the difficulty of the process.
The photolithography technique is complicate but the principle behind it is fairy simple. For example, photosensitive material is first coated on the wafer. Then, light source is projected to the photosensitive material through the mask, which is mainly composed of glass. The light beam through the mask has the same pattern as the mask so that the mask pattern can be transferred to the photosensitive material on the wafer. The photosensitive material can be, for example, photoresist. Then, after exposure or/and development, equal or complementary pattern on the mask can be transferred to the photosensitive material. If the photosensitive material is positive photoresist, it will obtain a pattern the same as the mask. On the other hand, if the photosensitive material is negative photoresist, it will obtain a complementary pattern as the mask.
As negative photoresist is used, after exposure and during the step of development, developer will penetrate into the space between the molecular in the negative photoresist.
Consequently, the photoresist swells and the differentiation between the after-development pattern of negative photoresist and the mask pattern increases. It is therefore not suitable for process down to 3 .mu.m to use negative photoresist. As a result, positive photoresist is more widely used to the sub-micron semiconductor technique presently.
The exposure technique used for transferring patterns from the mask to the photoresist has three main types: contact type, proximity type and projection type. For the semiconductor technique of deep sub-micron, the degree of precision of the exposure technique is highly limited to photo-resolution. The limitation of photo-resolution may cause error or even failure of pattern transferring. Proximity exposure technique is taken as an example. FIG. 1 illustrates the original mask pattern. After the step of exposure, the pattern of photoresist is substantially the same as the pattern shown in FIG. 2. Comparing the original mask pattern in FIG. 1 and the transferred pattern in FIG. 2, the difference between these two patterns is obvious. For example, the four sides 10, 11, 12, 13 of the original mask pattern shrinks to be sides 20, 21, 22, 23, as shown in FIG. 2, after the pattern is transferred. Due to the limitation of the resolution, the shrinking level of the narrower sides 10, 12 is larger than that of the wider sides 11, 13. Further, the configurations of the corners change greatly to become rounded corners. As shown in FIG. 1 and FIG. 2, the right-angled corner 14 becomes a rounded corner 24.
The problems coming after the error due to pattern transfer is serious. For example, during the back-end processes, the error may cause over-etching and improper coupling of interconnection.
Consequently, during the process of proximity pattern transfer, the original pattern is usually processed through optical proximity correction to form a pattern for transferring.
For example, if the original pattern is as shown in FIG. 3, in order to obtain better pattern transfer, assist features are generally added to form an optical proximity pattern as shown in FIG. 4. The assist features herein include assist features 40, 41, 42, 43, 44, 45, 46, 47. This optical proximity correction pattern provides a substantially similar transfer pattern of the original pattern after it is transferred. However, the sizes of the assist features are fairy small; as a result, they are hard to fabricate and suffer from the difficulties in mask inspection.